core: Fix incorrect substitution of env vars in directives containing multiple env vars.

In ap_resolve_env(), the string returned from getenv() should be copied since
the returned string may be statically allocated.

This fixes an issue where the value for the last env var is substituted for all
env vars in a directive containing multiple env vars.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1848236 13f79535-47bb-0310-9956-ffa450edef68